In Down syndrome , karyotyping has shown that the disorder is associated with trisomy of chromosome number 21 usually due to:

Options

A. Non-disjuction during egg-cell formation
B. Non-disjuction during sperm-cell formation
C. Non-disjunction during formation of egg-cell or sperm-cells
D. Addition of extra chromosome during mitosis of the zygote

Answer

To solve the question regarding Down syndrome and its association with trisomy of chromosome 21, we can break down the explanation into clear steps:

### Step-by-Step Solution:

1. **Understanding Down Syndrome**:
- Down syndrome is a genetic disorder characterized by the presence of an extra copy of chromosome 21, leading to a total of three copies instead of the usual two. This condition is known as trisomy 21.

2. **Karyotyping**:
- Karyotyping is a laboratory procedure that allows for the visualization of an individual's chromosomes. In individuals with Down syndrome, karyotyping reveals three copies of chromosome 21.

3. **Cause of Trisomy 21**:
- The primary cause of Down syndrome is nondisjunction, which is the failure of homologous chromosomes to separate properly during cell division.

4. **Meiosis and Nondisjunction**:
- Meiosis is the process of cell division that produces gametes (sperm and egg cells). During meiosis, homologous chromosomes normally segregate into different cells. This process is called disjunction.
- If nondisjunction occurs, it means that the homologous chromosomes do not separate, resulting in gametes that have an abnormal number of chromosomes.

5. **Formation of Gametes**:
- Nondisjunction can happen during the formation of either sperm or egg cells. If a gamete with an extra chromosome (in this case, chromosome 21) fuses with a normal gamete during fertilization, the resulting zygote will have three copies of chromosome 21.

6. **Conclusion**:
- Therefore, the disorder associated with Down syndrome (trisomy 21) is usually due to nondisjunction during the formation of either the egg cell or the sperm cell.

### Final Answer:
The disorder is associated with trisomy of chromosome number 21 usually due to nondisjunction during the formation of egg or sperm cells.

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